Halide compounds are conventionally used as flame retardants for imparting polymeric materials, such as epoxy resins and phenolic resins, with a flame-retardant property. However, to meet the requirements for environmental protection, the flame retardants for flame-retardant polymeric materials are required to be halogen-free. The flame retardants commonly used on the market are phosphorous-based flame retardants, such as phosphates and phosphonates. However, the phosphorous-based flame retardants usually have problems of inferior thermal stability and hydrolysis resistance. In order to solve the problems, various derivatives from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (referred to as DOPO hereinafter) have been developed.
In addition, in order to be useful in electronic materials of thermoset resins, the flame retardants should have a chemical group reactive with the thermoset resins, i.e., of the reactive type.
TW 572954 discloses a reactive type phosphorous-containing flame retardant having a chemical structure represented by

As shown in the chemical structure, the reactive type phosphorous-containing flame retardant disclosed in TW 572954 contains two phenyl groups each having a reactive group (i.e., OH) and two DOPO moieties. The phosphorous content of the reactive type phosphorous-containing flame retardant is merely 9.86 wt %. It should be noted that the flame-retardant effect of the reactive type phosphorous-containing flame retardant is directly proportional to the phosphorous content of the reactive type phosphorous-containing flame retardant.
U.S. Pat. No. 8,962,773 B2 discloses two reactive type phosphorous-containing flame retardants having chemical structures respectively represented by
and

Although the phosphorous contents of these two reactive type phosphorous-containing flame retardants are relatively high, 11.25 wt % and 10.86 wt %, respectively, compared to the reactive type phosphorous-containing flame retardant disclosed in TW 572954, the reactants for synthesizing them are not readily available. In addition, as shown in the two chemical structures above, the DOPO moiety is at an ortho position to the reactive —OH group, which may cause steric hindrance to the subsequent reaction of the reactive —OH group so that the reactivity of these two reactive type phosphorous-containing flame retardants may be relatively reduced.
U.S. Pat. No. 8,512,466 B2 discloses a phosphorous-containing oligomer useful as a reactive type flame retardant and having a chemical structure represented by

As shown in the chemical structure, the phosphorous-containing oligomer has a number n (n≧1) of phenyl groups each having a reactive group (i.e., OH), and a number n+1 of DOPO moieties. That is, the phosphorous-containing oligomer is not a pure compound. In addition, the phosphorous-containing oligomer is obtained by conducting two stages of reactions, i.e., a reaction at 80° C. to 180° C., followed by a reaction at 120° C. to 200° C.
U.S. Pat. No. 8,648,154 B2 discloses a phosphorous-containing phenol novolac resin prepared by subjecting 4-hydroxybenzaldehyde and DOPO to a reaction (Reaction 1) to produce DOPO-HB, and subjecting DOPO-HB and an aldehyde to a further reaction (Reaction 2).
